Marine vessels perform a variety of tasks, one of which is the handling of anchors. This is particularly important in the offshore oil and gas industry where very large drilling rigs are kept in position by a plurality of anchors. Because of their size and the nature of the anchoring procedures, these drilling rigs require assistance from other vessels in anchor handling procedures.
The term "anchor handling" describes a wide variety of operations including the following: receiving the anchor from the rig, carrying the anchor to its proper location and deploying it, extracting the anchor from the sea bottom and hauling it to the surface, bringing the anchor on board the retrieving vessel, and carrying the anchor back to the rig. Some anchor handling procedures involve all of the above, while other procedures involve only some of the above.
There are basically two types of marine vessels which assist the rig in anchor handling, the tub and the supply-tug. The latter resembles the traditional tug except that it has an extended rear deck section.
This extra deck space makes the supply-tug more suitable than the tug for those anchor handling operations involving the boarding of anchors. When an anchor is brought on board, even in moderately calm seas, the anchor is jostled about the deck. The supply-tug deck provides ample room for the anchor to move about without becoming entangled with the men and equipment located at the forward deck of the vessel. Additionally, the extra deck space on the supply-tug provides necessary storage for the anchor as well as other items associated with anchors such as anchor buoys and cables.
The usual method of extracting a drilling rig anchor from the sea floor involves the use of a cable called a pennant line. One end of the pennant line is attached to the anchor. The other end is attached to the anchor's marker buoy.
When the anchor is in place in the sea floor, the pennant line is slack, the buoy being allowed to float relatively free. When the anchor is to be extracted from the sea floor, the retrieving vessel locates the buoy and brings it on board. The pennant line which was attached to the buoy is then attached to a winch located approximately in the center of the vessel and the slack is taken out of the line. The next step involved in extracting the anchor is known as "breaking anchor".
In this step, a tremendous pulling force is gradually applied to the anchor via the pennant line until the anchor is freed from the seabed. Because of the tremendous tension in the pennant line while breaking anchor, it is important to keep the pennant line centered at the vessel's stern. Centered thusly, the vessel's engines are used more efficiently and there is less danger of the vessel capsizing. After breaking anchor and while hauling the anchor to the surface, it is desirable to maintain the pennant line centered at the vessel's stern instead of allowing the line to rub back and forth across the stern.
The traditional way of centering the pennant line has been to use a pair of vertical pins located at the center of the vessel's stern in combination with a winch situated at the vessel's center, forward of the stern.
Vertical pins are employed on both tugs and supply-tugs while breaking and hauling in the anchor. Because the supply-tug is used for jobs not normally performed by the tug, particularly the boarding of anchors, there has been a divergence in the development of vertical pins used on tugs as compared with those vertical pins used on supply-tugs. Due to the weight of the anchor and in order to avoid interference with the anchor, the vertical pins used on a supply-tug must be capable of "disappearing" i.e. there must be no manifestation of their presence on deck immediately prior to bringing the anchor on board.
Vertical pins generally used today on a supply-tug are best described as dowels which fit into cylindrical cavities located at the stern beneath the deck. Many such vertical pins are manually inserted and manually removed from the cavity. Some supply-tugs are equipped with hydraulically operated pins which are normally stored inside their cylindrical cavity, the top of the pin being flush with the deck. The pins are hydraulically raised and lowered as needed.
There have been problems associated with the dowel-cylindrical cavity type of vertical pin used on supply-tugs. Often, when such pins are in their raised position, the pennant line is pulled very strongly against one of the pins. When subjected to such a horizontal load the pin, being supported at only one end, often becomes bent. Such bending causes the dowel to become stuck in its cavity. So stuck, the pin is a very aggravating and dangerous nuisance because the anchor must either be pulled over the obstructing pin or the anchor boarding procedure must be delayed until the deformed pin is cut off. Also, because this type of vertical pin requires a very close fit between the dowel and the cavity, rust and debris have been known to cause the dowel to stick in its cavity.
These problems have generally not been encountered with the vertical pins used on tugs. Because a tug normally doesn't board anchors, there is no need to have pins which are capable of "disappearing" from the deck. This means the vertical pin can be sufficiently strengthened against deformation by various frames and supporting devices securely fixed to the deck.